Immune complex-mediated activation of Fc gamma receptors (FcgammaR) on inflammatory cells plays a critical role in the pathogenesis of autoimmune diseases. Fc receptors for IgG (FcgammaRI, FcgammaRIII), IgE (FcepsilonRI) and IgA (FcalphaRI) exhibit a high degree of conservation in signaling including utilization of the same signaling subunits (FcRgamma chains), activation of the same non-receptor tyrosine kinases (NRTK) (syk), and phosphorylation of common downstream substrates (PLCgamma1,cbl, shc). CD81 has been identified as a membrane antigen recognized by antibodies which inhibit FcepsilonRI- and FcgammaRIII- mediated degranulation in mast cells. Since inflammatory cells express both CD81 and FCgammaRs, but not FcepsilonRI, it is reasonable to assume that CD81 could also inhibit FcgammaR- signaling in these cells. The overall objectives of this project are to determine the extent of CD81 inhibition of Fc receptor signaling and the mechanism by which CD81 inhibits FcepsilonRI and FcgammaRIII signaling. Three lines of investigation will be employed to meet these objectives. First, CD81- mediated effects on FcgammaR signaling events such as phagocytosis, cytokine synthesis and degranulation will be examined in normal inflammatory cells and lines. Second, since phosphatases regulate Fc and other antigen receptor signaling, the role of tyrosine phosphatases as effectors of CD81 signaling will be assessed. In addition, CD81- mediated effects on late events of FcR-signaling (ras/MAP kinase, focal adhesion kinase activation) as well as CD81-mediated effects on cell matrix adhesion receptors such as VLA4, will comprise a major part of this line of investigation. Third, further experiments will examine the effect of pretreatment of mice with CD81 antibodies in vivo prior to initiation of IgE- and immune complex-mediated Arthus and anaphylactic reactions. The proposed studies address the extent of CD81 inhibition in FcepsilonRI- and FcgammaR function, its mechanism of action and the potential role of CD81 as an inhibitor of immune complex-mediated activation in vivo, clarification of the role of CD81 in FcgammaR signaling will provide pertinent information as to how FcgammaR activation events are regulated in normal and disease conditions.